Warrington was a Hogwarts student in Slytherin House from 1989-1996. He was a Quidditch team Chaser and member of the Inquisitorial Squad put into place in the school by Professor Dolores Umbridge.
Warrington is two years older than Harry Potter. He was described by Dean Thomas as:
“That big bloke from Slytherin who looks like a sloth….” (GF16).
Warrington joined the Slytherin Quidditch Team in 1993 (PA15) and unsuccessfully entered himself for the Triwizard Tournament in 1994 (GF16).
While a member of the Inquisitorial Squad (and coincidentally just before a big match with Gryffindor), Warrington suffered an attack that left him with a horrible skin ailment which made him look as though he had been coated in cornflakes (OP30).
He was one of the Inquisitorial Squad who caught Ron Weasley, Ginny Weasley, Neville Longbottom and Luna Lovegood while Harry was trying to contact Sirius Black in Umbridge’s fireplace (OP32).
Other canon notes and references
He must have been at least seventeen on October 30 1994, to have put his name in the Goblet of Fire. Given that he was still at school the next year, he presumably started at Hogwarts in 1989 (unless he had to retake a year).
Commentary
Etymology
Warrington is a surname of Anglo-Saxon origin, coming from either the town of Warrington in Buckinghamshire or the one in Lancashire. It derives from Olde English waer "pledge", heard "hardy, brave, strong" and tun "homestead, settlement" (Internet Surname Database).
Notes
He is probably the same as the C. Warrington who checked Quidditch Through the Ages out of the Hogwarts school library (13 November) (QA).
There could be a link between Warrington's cornflake-like skin complaint and the snuffbox of Wartcap powder from No. 12 Grimmauld Place, that gave Sirius's hand an 'unpleasant crusty covering' and that George Weasley subsequently pocketed (OP6).
According to the Harry Potter Wiki, Warrington's first name is Cassius. This detail was on a film prop in Harry Potter Wizard's Collection. While this does match the first initial seen in Quidditch Through the Ages, it cannot be considered canon.
Lexicon list of Chasers